If I insert a finger in my girlfriend’s anus, will it get dirty?

No, that is not to be expected. The anal canal and rectum contain little or no faeces. Faeces pass through it when your girlfriend has a bowel movement but are not stored there. Exception to this rule is when your girlfriend is used to ignore the rectal reflex and doesn’t go to the bathroom when she actually needs to. Bathing beforehand is normally enough to clean the anal area. To make sure there are no faeces in the rectum some people clean it before having sex by using an enema. But this is not really necessary.